1. Field of the Invention
The invention relates to telecommunications. More particularly, the invention relates to a communications interface bus which can carry synchronous and asynchronous data signals simultaneously.
2. State of the Art
The first commercial digital voice communications system was installed in 1962 in Chicago, Ill. The system was called “T1” and was based on the time division multiplexing (TDM) of twenty-four telephone calls on two twisted wire pairs. The T1 system is still widely used today and forms a basic building block for higher capacity communication systems including T3 which transports twenty-eight T1 signals. The designations T1 and T3 were originally coined to describe a particular type of carrier equipment. Today T1 and T3 are often used to refer to a carrier system, a data rate, and various multiplexing and framing conventions. It is more accurate to use the designations “DS1” and “DS3” when referring to the multiplexed digital signal carried by the T1 and T3 carriers, respectively.
Today, another higher bandwidth TDM system is in use. This system is referred to as the synchronous optical network (SONET) or, in Europe, the synchronous digital hierarchy (SDH). The SONET network is designed to provide enormous bandwidth. SONET signals are referred to as Synchronous Transport Signals (STS) or Optical Carriers (OC). The narrowest SONET signal is referred to as STS-1 or OC-1. It has a bandwidth of 51.84 Mb/s which is sufficient to carry twenty-eight DS1 signals or a single DS3 signal. The hierarchy includes STS-3 (OC-3) which is three times the bandwidth of an STS-1 (OC-1) signal, and higher bandwidth signals increasing in multiples of four, i.e. STS-12 (OC-12), STS-48 (OC-48), STS-192 (OC-192), and STS-768 (OC-768).
SONET signals are said to be synchronous because all nodes in the SONET network are synchronized to a common reference clock. The older T-1 and T-3 signals are said to be plesiochronous (nearly synchronous) because the clock rate of each signal is tightly controlled. As used herein, however, all signals other than SONET signals are referred to as asynchronous. When multiple T-1 or T-3 signals from different sources are demultiplexed from a SONET signal, each of these plesiochronous signals will have its own separate clock. The clock for each signal is derived from the SONET signal using a desynchronizer. For example, when demultiplexing an STS-1 signal which carries twenty-eight DS-1 data signals, twenty-eight desynchronizers will be used to create twenty-eight separate clock signals, one for each DS-1 signal. In fact, each DS-1 signal will have its own clock, data, and frame signals.
As telecommunication networks grow, it is desirable to miniaturize network equipment as much as possible, particularly in urban areas where space is expensive and networks are large. For example, a SONET demultiplexer may be made of a few integrated circuit chips. One of the difficulties in miniaturization is that as more devices are placed on a single chip, more leads or pins are required. The physical size of the chip limits the number of leads which can be used. For example, in a SONET demultiplexer, it would be necessary to provide three pins (clock, data, and frame) for each “possible” asynchronous signal. By “possible”, it is meant that in a higher order SONET signal, such as an STS-3, demultiplexing may involve separating out twenty-eight DS-1 signals and two STS-1 signals or it could involve separating out up to eighty-four DS-1 signals. Thus, a demultiplexing solution for an STS-3 would require eighty-four data lines, eight-four clock lines, and eight-four frame signal lines in order to terminate eighty-four DS-1 signals. This would require many separate pins on a chip.